Field of the Invention
The present invention relates to a fixing device included in an image forming apparatus such as an electrophotographic copying machine and printer, and a heater used in the fixing device.
Description of the Related Art
As a fixing device included in an image forming apparatus such as a copying machine and a laser beam printer, one using a film is known. Such a fixing device typically includes a cylindrical film, a plate-shaped heater which makes contact with an inner surface of the film, and a pressure member which forms a nip portion with the heater via the film. The fixing device performs fixing processing at the nip portion while conveying and heating a recording material having a toner image formed thereon to fix the toner image to the recording material.
The fixing device uses a film having a low heat capacity. The fixing device thus has an advantage of a short warm-up time, which contributes to reduced first print out time (FPOT) of the image forming apparatus. However, if small-sized sheets are continuously printed, a phenomenon in which an area of the nip portion where the recording materials do not pass rises in temperature, or a temperature rise of a non-sheet passing are, is likely to occur.
As a technique for suppressing the temperature rise of the non-sheet passing area, there is known a heater including a substrate on which a heat generation resistor having a positive resistance-temperature characteristic (positive temperature coefficient (PTC) characteristic) is formed. If a current is applied to a heat generation resistor having a high PTC characteristic in a conveyance direction of a recording material, the resistance of a sheet non-passing portion that rises in temperature increases. This can reduce the current flowing through the heat generation resistor and then reduce the amount of heat generation in the sheet non-passing portion, thereby suppressing the temperature rise of the non-sheet passing area.
The heat generation resistor is made of a paste material. Since paste materials having a high PTC characteristic have low sheet resistance, the amount of heat generation needed for the heater used in the fixing device may be difficult to obtain. Japanese Patent Application Laid-Open No. 2012-189808 discusses a heater that includes a plurality of longitudinally-divided conductive patterns connected to a heat generation resistor along a longitudinal direction. Such a heater can provide a total resistance needed for the heater used in the fixing device while using a paste material having a low sheet resistance.
However, the heater discussed in Japanese Patent Application Laid-Open No. 2012-189808 has a problem that the amount of heat generation drops locally in an area corresponding to a gap between the conductive patterns of the heater, possibly causing temperature variations of the heater in the longitudinal direction.